In the field of multi-level demodulators such as quadrature amplitude modulation (QAM) demodulators, it is known to provide a VCO circuit and a control loop to establish phase synchronization between the VCO and a received radio frequency carrier signal. This is advantageous for demodulation purposes.
A received signal is downmixed to a suitable intermediate frequency (I.F.) signal and is further downconverted to baseband by mixing with the VCO signal. For the result to be converted to digital form and to be demodulated into in-phase (I) and quadrature (Q) samples, locking of the VCO to the received carrier signal is necessary for coherent detection of M-ary quadrature amplitude modulated signals.
It is a disadvantage in existing circuits that phase or frequency comparators have a predefined range of operation. If the difference between the received signal and the reference signal is outside this range, the circuit is hopelessly unable to achieve lock. In the past it has been the practice always to ensure that the operation of the VCO is within the range of operation of the comparator. This has been achieved by presteering the VCO with trimmable resistors and capacitors which are settable in the factory to match the particular range of operation of the detector of the circuit and the particular VCO at its normal operating temperature.
By careful calibration, the circuit can be set such that on start-up the VCO always oscillates at a frequency which is offset from the wanted carrier frequency by no more than the predefined operating range of the detector. This is expensive in terms of technician time in performing the calibration and trimming the components.
Selecting a more accurate or higher stability VCO is not an option as this too involves expense or the accuracy and stability of the VCO may not be the limiting factor in the circuit.
Selecting a wider range of operation detector is not an option. For example in the case of a phase detector the maximum predefined range of operation is 360 degrees of the reference frequency signal. Selecting a wider range has an impact on the speed of locking or the stability of the loop.
The speed of operation of the loop is a significant design factor. A higher locking speed introduces greater instability while a lower locking speed can be unacceptable.
There is a need for an arrangement which avoids or mitigates some or all of the above deficiencies.